If you want to send an email on data change of that attribute you can build an observer on the customer_save_after Event, were you want to check the old value and the newest one of the customer and if that changed from NO to YES since this will be triggered every time you'll save the customer.
I finally figured it out.
When you add an item to your cart it then directs you to /checkout/chart/ and it rebuilds the cart using a query to sales_flat_quote_item. If you have two products with the same product ID, each item in your quote actually will contain a reference to the same product object. This was confusing because when I first added my products ...